1. Field of the Invention
The present invention relates to an image reading device and an image reading method that are used in reading visible images and invisible images, and a light source for the same.
2. Description of the Related Art
A printing technique using an ink or toner that transmits visible lights and absorbs invisible lights is becoming popular. Invisible images acquired by such a technique can be read by the analysis of the invisible lights reflected therefrom. Even a regular image reading device is desired to adopt the reading function for such invisible lights. Accordingly, various techniques regarding a single device that can read both the visible images and the invisible images have already been put forward.
There is, for example, a technique proposed such that a visible light source and an infrared light source are individually provided in a single device. Such a technique, however, necessarily involves the size expansion of the device. Further, if multiple light sources are used, there are inevitable differences in positions in which the light sources each emit, which creates shades on the copy, so that precise reading will become difficult.
On the other hand, in the technique disclosed in the Japanese Published Unexamined Patent Application No. Hei 6-141145, a halogen lamp having a wide wavelength band of emission which emits a visible light and an infrared light is employed, either a visible light cutoff filter or an infrared cutoff filter is selectively positioned in the path of a reflected light from the copy image, whereby both the visible images and the invisible images can be read. This technique, however, requires a halogen lamp with great power consumption, which emits a light of a wide wavelength band.
In recent years, for the purpose of reducing the power consumption and enhancing the reliability, as a light source for the general image reading device that reads the visible images, a fluorescent lamp (specially, a rare gas fluorescent lamp having high stability of luminous energy) is increasing in use, instead of the halogen lamp. It is convenient to use a fluorescent lamp as a light source to read not only the visible images but also the invisible images. However, a fluorescent lamp scarcely emits an invisible light such as an infrared light under the condition of emitting a visible light efficiently.